Immune System and Cancer
Malignant tumors are inherently resistant to conventional therapies and present significant therapeutic challenges. Immunotherapy has become an evolving area of research and an additional option for the treatment of certain types of cancers. The immunotherapy approach rests on the rationale that the immune system may be stimulated to identify tumor cells, and target them for destruction.
Numerous studies support the importance of the differential presence of immune system components in cancer progression [1]. Clinical data suggest that high densities of tumor-infiltrating lymphocytes are linked to improved clinical outcome [2]. The correlation between a robust lymphocyte infiltration and patient survival has been reported in various types of cancer, including melanoma, ovarian, head and neck, breast, urothelial, colorectal, lung, hepatocellular, gallbladder, and esophageal cancer [3]. Tumor immune infiltrates include macrophages, dendritic cells (DC), mast cells, natural killer (NK) cells, naïve and memory lymphocytes, B cells and effector T cells (T lymphocytes), primarily responsible for the recognition of antigens expressed by tumor cells and subsequent destruction of the tumor cells by T cells.
Despite presentation of antigens by cancer cells and the presence of immune cells that could potentially react against tumor cells, in many cases the immune system does not get activated. Key to this phenomenon is the ability of tumors to protect themselves from immune response by coercing cells of the immune system to inhibit other cells of the immune system. For example, CD4+ T cells possess the ability to differentiate into T regulatory (Treg) cells, which have the ability to inhibit activated T cells. Additionally, cancer cells can impair CD8+ T cell effector function, leading to the evasion of anti-tumor immune response. Finally, the local immunosuppressive nature of the tumor microenvironment, along with immune editing, can lead to the escape of cancer cell subpopulations that do not express the target antigens. This, finding a method to that would allow for the preservation and/or restoration of anti-tumor activities of the immune system is of paramount importance.
It has been established that type I IFN plays important roles in host antitumor immunity [4]. IFNAR1-deficient mice are more susceptible to developing tumors after implantation of tumor cells. Spontaneous tumor-specific T cell priming is also defective in IFNAR1-deficient mice [5, 6]. More recent studies have shown that the cytosolic DNA-sensing pathway is important in the recognition of tumor-derived DNA by the innate immune system. In turn, this leads to the development of antitumor CD8+ T cell immunity [7]. This pathway also plays an important role in radiation-induced antitumor immunity [8].
Melanoma
Melanoma, one of the deadliest cancers, is the fastest growing cancer in the US and worldwide. Its incidence has increased by 50% among young Caucasian women since 1980, primarily due to excess sun exposure and the use of tanning beds. According to the American Cancer Society, approximately 76,380 people in the US will be diagnosed with melanoma and 10,130 people (or one person per hour) are expected to die of melanoma in 2016. In most cases, advanced melanoma is resistant to conventional therapies, including chemotherapy and radiation. As a result, people with metastatic melanoma have a very poor prognosis, with a life expectancy of only 6 to 10 months. The discovery that about 50% of melanomas have mutations in BRAF (a key tumor-promoting gene) opened the door for targeted therapy in this disease. Early clinical trials with BRAF inhibitors showed remarkable, but unfortunately not sustainable responses in patients with melanomas with BRAF mutations. Therefore, alternative treatment strategies for these patients, as well as patients with melanoma without BRAF mutations, are urgently needed.
Human pathological data indicate that the presence of T-cell infiltrates within melanoma lesions correlates positively with longer patient survival [9]. The importance of the immune system in protection against melanoma is further supported by partial success of immunotherapies, such as the immune activators IFN-α2b and IL-2 [10] as well as the unprecedented clinical responses of patients with metastatic melanoma to immune checkpoint blockade therapy, including anti-CTLA-4 and anti-PD-1/PD-L1 used either individually or in combination [11-17]. However, many patients fail to respond to immune checkpoint blockade therapy alone. The addition of virotherapy might overcome resistance to immune checkpoint blockade, which is supported by animal tumor models [18].
Poxviruses
Poxviruses, such as engineered vaccinia viruses, are in the forefront as oncolytic therapy for metastatic cancers [19]. Vaccinia viruses are large DNA viruses, which have a rapid life cycle [20]. Poxviruses are well suited as vectors to express multiple transgenes in cancer cells and thus to enhance therapeutic efficacy [21]. Preclinical studies and clinical trials have demonstrated efficacy of using oncolytic vaccinia viruses and other poxviruses for treatment of advanced cancers refractory to conventional therapy [22-24]. Poxvirus-based oncolytic therapy has the advantage of killing cancer cells through a combination of cell lysis, apoptosis, and necrosis. It also triggers the innate immune sensing pathway that facilitates the recruitment of immune cells to the tumors and the development of anti-tumor adaptive immune responses. The current oncolytic vaccinia strains in clinical trials (JX-594, for example) use wild-type vaccinia with deletion of thymidine kinase to enhance tumor selectivity, and with expression of transgenes such as granulocyte macrophage colony stimulating factor (GM-CSF) to stimulate immune responses [21]. Many studies have shown however that wild-type vaccinia has immune suppressive effects on antigen presenting cells (APCs) [25-28] and thus adds to the immunosuppressive and immunoevasive effects of the tumors themselves.
Poxviruses are extraordinarily adept at evading and antagonizing multiple innate immune signaling pathways by encoding proteins that interdict the extracellular and intracellular components of those pathways [29]. Modified vaccinia virus Ankara (MVA) is an attenuated vaccinia virus that was developed through serial passaging in chicken embryonic fibroblasts. MVA has a 31-kb deletion of the parental vaccinia genome and was used successfully as a vaccine during the WHO-sponsored smallpox eradication campaign [30-32]. MVA has been investigated intensively as a vaccine vector against HIV, tuberculosis, malaria, influenza, and coronavirus, as well as cancers [33-38].
MVA has deletions or truncations of several intracellular immunomodulatory genes including K1L, N1L, and A52R, which have been implicated in regulating innate immune responses [39-46]. On the other hand, MVA retains the E3L gene encoding a bifunctional Z-DNA/dsRNA binding protein, a key vaccinia virulence factor [47-55]. It has been shown that MVA infection of human monocyte-derived dendritic cells causes DC activation [56]. Waibler et al. [57] reported that MVA infection of murine Flt3L-DC triggered a TLR-independent type I IFN response. In addition, MVA infection of human macrophages triggers type I IFN and pro-inflammatory cytokines and chemokines via a TLR2/TLR6/MyD88 and MDA5/MAV5-dependent pathways [58].
The sensing of DNA in the cytosol triggers a cascade of events leading to the production of type I IFN and cytokines as well as cellular stress responses. STING (stimulator of IFN genes) was identified as an important adaptor for the cytosolic DNA-sensing pathway [59-61]. The nature of the DNA sensors remained elusive until the discovery of cyclic GMP-AMP synthase (cGAS) as the critical DNA sensor, and its product cyclic GMP-AMP, which contains an unanticipated 2′,5′ linkage at the GpA step and standard 3′,5′ linkage at the ApG step [62-68]. Subsequent research confirmed STING as the key adaptor activated by cGAMP, thereby mediating the cascade of downstream events involving kinases and transcription factors that lead to the interferon response [66, 68, 69]. We reported that MVA infection of murine conventional dendritic cells induces type I IFN via a cytosolic DNA-sensing pathway mediated by cytosolic DNA sensor cGAS, its adaptor STING, and transcription factors IRF3 and IRF7. By contrast, wild-type vaccinia virus fails to activate this pathway. Intravenous inoculation of MVA via tail-vein injection induced type I IFN secretion in WT mice, which was diminished in STING or IRF3-deficient mice [70]. Furthermore, we showed that vaccinia virulence factors E3 and N1 play inhibitory roles in the cytosolic DNA-sensing pathway [70].